Download 10.1 The Basic Energy Model

10.1 The Basic Energy Model ­ 10.2 Work.notebook
January 03, 2017
The Basic Energy Model
Every system in nature has a quantity we call its total energy E.
Section 10.1 The Basic Energy Model
Energy Transformations
Forms of Energy
Some important forms of energy are
> Kinetic energy K: energy of motion.
> Gravitational potential energy Ug: stored energy associated with an object’s height above the ground.
> Elastic or spring potential energy Us: energy stored when a spring or other elastic object is stretched.
> Thermal energy Eth: the sum of the kinetic and potential energies of all the molecules in an object.
> Chemical energy Echem: energy stored in the bonds between molecules.
> Nuclear energy Enuclear: energy stored in the mass of the nucleus of an atom.
Energy Transformations
Energy of one kind can be transformed into energy of another kind within a system.
QuickCheck 10.1
A child is on a playground swing, motionless at the highest point of his arc. What energy transformation takes place as he swings back down to the lowest point of his motion?
The weightlifter converts chemical energy in her body into gravitational potential energy of the barbell.
Elastic potential energy of the springboard is converted into kinetic energy. As the diver rises into the air, this kinetic energy is transformed into gravitational potential energy. A. K → Ug
B. Ug → K
C. Eth → K
D. Ug → Eth
E. K → Eth
10.1 The Basic Energy Model ­ 10.2 Work.notebook
January 03, 2017
Energy Transfers and Work
QuickCheck 10.1
A child is on a playground swing, motionless at the highest point of his arc. What energy transformation takes place as he swings back down to the lowest point of his motion?
A. K → Ug
B. Ug → K
C. Eth → K
D. Ug → Eth
E. K → Eth
Energy Transfers and Work
• Energy can be transferred between a system and its environment through work and heat.
• Work is the mechanical transfer of energy to or from a system by pushing or pulling on it.
• Heat is the nonmechanical transfer of energy between a system and the environment due to a temperature difference between the two.
The Work­Energy Equation
• Work represents energy that is transferred into or out of a system. • The total energy of a system changes by the amount of work done on it.
The athlete does work on the shot, giving it kinetic energy, K.
The hand does work on the match, giving it thermal energy, Eth.
The boy does work on the slingshot, giving it elastic potential energy, U s.
• Work can increase or decrease the energy of a system. • If no energy is transferred into or out of a system, that is an isolated system.
The Law of Conservation of Energy
The total energy of an isolated system remains constant.
Section 10.2 Work
10.1 The Basic Energy Model ­ 10.2 Work.notebook
Work
January 03, 2017
Calculating Work
Work is done on a system by external forces: forces from outside the system.
• Although both the force and the displacement are vectors, work is a scalar. • The unit of work (and energy) is:
Example 10.1 Work done in pushing a crate
Sarah pushes a heavy crate 3.0 m along the floor at a constant speed. She pushes with a constant horizontal force of magnitude 70 N. How much work does Sarah do on the crate?
Example 10.1 Work done in pushing a crate (cont.)
prepare We begin with the before­and­after visual overview in FIGURE 10.6. Sarah pushes with a constant force in the direction of the crate’s motion, so we can use Equation 10.5 to find the work done.
solve The work done by Sarah is
W = Fd = (70 N)(3.0 m) = 210 J
By pushing on the crate Sarah increases its kinetic energy, so it makes sense that the work done is positive.
Force at an Angle to the Displacement
• Only the component of a force in the direction of displacement does work.
• If the force is at an angle θ to the displacement, the component of the force, F, that does work is Fcosθ.
Force at an Angle to the Displacement
10.1 The Basic Energy Model ­ 10.2 Work.notebook
Force at an Angle to the Displacement
January 03, 2017
Force at an Angle to the Displacement
The sign of W is determined by the angle θ between the force and the displacement. Example 10.2 Work done in pulling a suitcase
A strap inclined upward at a 45° angle pulls a suitcase through the airport. The tension in the strap is 20 N. How much work does the tension do if the suitcase is pulled 100 m at a constant speed?
Example 10.2 Work done in pulling a suitcase (cont.)
prepare figure 10.8 shows a visual overview. Since the suitcase moves at a constant speed, there must be a rolling friction force (not shown) acting to the left.
solve We can use Equation 10.6 , with force F = T, to find that the tension does work:
W = Td cos θ = (20 N)(100 m)cos 45° = 1400 J
The tension is needed to do work on the suitcase even though the suitcase is traveling at a constant speed to overcome friction. So it makes sense that the work is positive. The work done goes entirely into increasing the thermal energy of the suitcase and the floor.
Forces That Do No Work
A force does no work on an object if
> The object undergoes no displacement.
> The force is perpendicular to the displacement.
> The part of the object on which the force acts undergoes no displacement (even if other parts of the object do move). Text: p. 291